Why can't I access my panel's getWidth() and getHeight() functions?

Question

I\'m writing a simple program to test out basic GUI. The program prints a letter in the middle of the screen and allows the user to move it with the arrow keys. Everything w

Answer 1

Swing components have no width or height until they've been rendered. This occurs if you call pack() or setVisible(true) on a root container. Consider placing your x y int initialization code in the componentResized method of a ComponentListener that is added to your JPanel.

e.g.,

import java.awt.event.*;
import java.awt.*;

import javax.swing.*;

public class TestLinePanel {
   private static void createAndShowGui() {
      LinePanel mainPanel = new LinePanel();

      JFrame frame = new JFrame("TestLinePanel");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }

   static class LinePanel extends JPanel {
      private static final int PREF_W = 400;
      private static final int PREF_H = PREF_W;
      private char keyChar = 'A';
      private int x;
      private int y;
      private boolean xySet = false;

      public LinePanel() {
         setFocusable(true);
         addKeyListener(new KeyAdapter() {
            public void keyPressed(KeyEvent e) {
               switch (e.getKeyCode()) {
               case KeyEvent.VK_DOWN:
                  y += 10;
                  break;
               case KeyEvent.VK_UP:
                  y -= 10;
                  break;
               case KeyEvent.VK_LEFT:
                  x -= 10;
                  break;
               case KeyEvent.VK_RIGHT:
                  x += 10;
                  break;
               default:
                  keyChar = e.getKeyChar();
               }

               repaint();

            }
         });

         addComponentListener(new ComponentAdapter() {

            @Override
            public void componentResized(ComponentEvent e) {
               if (!xySet) {
                  int xCenter = getWidth() / 2;
                  int yCenter = getHeight() / 2;

                  x = xCenter;
                  y = yCenter;

                  requestFocusInWindow();
                  xySet = true;
               }
            }

         });
      }

      @Override
      public Dimension getPreferredSize() {
         return new Dimension(PREF_W, PREF_H);
      }

      protected void paintComponent(Graphics g) {
         super.paintComponent(g);
         g.setFont(new Font("TimesRoman", Font.PLAIN, 24));
         g.drawString(String.valueOf(keyChar), x, y);
      }
   }
}

You'll also want to use key bindings rather than a KeyListener to capture your key strokes.